A major consideration in the design of power semiconductor packages is the manner of mounting the semiconductor elements for efficient thermal dissipation to prevent overheating and possible destruction of the semiconductors. High power packages of various configurations are known for the thermal and electrical packaging of power semiconductors, one known form being known as a TO-3 package. The conventional TO-3 package has a header plate with a cylindrical section and a mounting flange, with the electrical leads extending through the bottom mounting surface of the header. The leads are glass sealed to the header and are electrically insulated therefrom, and extend outwardly from the mounting surface. One or more semiconductor die assemblies are mounted on the header within the cylindrical section, and a cover is welded over this section to enclose the semiconductors. The holes in the mounting surface, through which the leads extend and are sealed, interrupt the mounting surface and lessen the thermal contact area of that surface. As a result, the thermal dissipation is decreased in comparison to that of a continuous mounting surface. The conventional TO-3 package requires double-sided mounting wherein the package is mechanically mounted on one side of the heat sink, while the electrical connections must be made on the other side.